Muffler units used in general-purpose engines are known as disclosed, for example, in Japanese Utility Model Publication (JP-UM-A) No. 2-99214, Japanese Patent Publication (JP-A) No. 2003-097289, and Japanese Utility Model Publication (JP-UM-A) No. 59-194521.
FIG. 17 hereof illustrates a muffler unit disclosed in JP-UM-A 2-99214. As shown in this figure, the disclosed muffler unit includes a muffler 300 having a pair of housing members 301 and 302 connected together with a partition wall 303 disposed therebetween. The muffler 300 has two sound-deadening chambers 304, 305 defined therein on opposite sides of the partition wall 303. The housing member 304 has an inlet opening 308 through which the first chamber 304 communicates with an exhaust port 307 of a general-purpose engine 306. The partition wall 303 has a dome-like recessed portion 311 facing the inlet opening 308, and a plurality of lancet holes 312 through which the first and second sound-deadening chambers 304, 305 communicate with each other. The second sound-deadening chamber 305 is open to the outside air through a tail pipe 109
With the muffler unit thus arranged, when exhaust gasses are routed to pass through the first and second chambers 304, 305 via the lancet holes 312, the speed and pressure of the gasses drop and the sound level is reduced. In this instance, however, since the dome-like recessed portion 311 is formed as an integral part of the partition wall 303 disposed inside the muffler 300, it is not expected at all for the muffler 300 to provide substantive reduction of exhaust sound by performing effective cooling of exhaust gasses while the gasses pass through the muffler 300.
FIG. 18 hereof illustrates a muffler unit disclosed in JP-UM-A 59-194521. As shown in this figure, the muffler unit includes a muffler 333 and an exhaust pipe 334 connected at one end to the muffler 333 and at the other end to an exhaust port 332 of a general purpose engine 311. The exhaust pipe 334 has an intermediate portion secured by a pair of screws 335 to a portion of the engine 311 including the exhaust port 332.
With this arrangement, since the muffler 333 is supported by the exhaust pipe 334 in a cantilevered fashion, the muffler 333 is likely to cause undue oscillation, which may generate unpleasant oscillation noise. An attempt may be made to increase the rigidity of the exhaust pipe 334 and a joint between the exhaust pipe and the engine body, however, attempted strengthening would increase the overall weight and size of the general-purpose engine 311 including the muffler unit.
FIG. 19 hereof illustrates a muffler unit disclosed in JP-A 2003-097289. As shown in this figure, the muffler unit includes a muffler 343 and an exhaust pipe 344 connected at one end to the muffler 343 and at the other end to an exhaust port 342 of a general purpose engine 341. The muffler 343 is also supported in a cantilevered fashion by the exhaust pipe 344 and hence is likely to undergo oscillating movement about the proximal end of the exhaust pipe 344 secured to the engine body. Furthermore, the muffler 343 is disposed above a cylinder block 347 of the engine 341 and is secured to a distal end of the exhaust pipe 344 by means of screws (not shown) extending in a vertical plane. With this arrangement, there is provided between the muffler 343 and the cylinder block 347 only a small space which is available for attachment of the muffler 343 and the exhaust pipe 344 by means of the non-illustrated screws. Thus, the efficiency of a muffler mounting operation using such small space is very low.
In view of the foregoing difficulties of the conventional device, it is desirable to provide a muffler unit for a general-purpose engine, which is able to perform effective cooling of exhaust gasses and thus achieving substantive reduction of the exhaust sound while the gasses are routed to pass through the muffler, is unlikely to cause undue oscillation which would otherwise result in generation of unpleasant oscillation noise, and can be attached to the engine body with increased efficiency.